Patent GB-1370180-A (GB)
Title: Apparatus for fault testing binary circuit subsystems
IPC: G01R31/319
Filed: 1972-10-25
Abstract: 1370180 Logic circuit tester BURROUGHS CORP 25 Oct 1972 [8 Nov 1971] 49212/72 Heading G1U A logic circuit testing system of the type in which a sequence of test words is applied to a logic circuit under test, is characterized in that sequence of test words is determined both by the particular construction of a test word generator and by the output signals produced by the circuit under test as the sequence of test words is applied to it, these output signals being injected, at appropriate points, into the test word generator. The word being produced by the test word generator after it has been driven through a sequence of a fixed number of test words is then compared with a reference word, produced by a known to be good circuit under similar conditions, to provide an overall test of whether a fault exists anywhere within the circuit under test. Once the existence of a fault has been established the system is put into a 'fault locate' mode of operation in which the test word generator is again driven through a sequence of a fixed number of test words, but with the feedback from the output of the circuit under test to the test word generator inhibited, and the number of onebits occurring on each output pin of the circuit under test is counted. Any count differing from the corresponding count produced by a known to be good circuit then indicates that a fault exists within that region of the circuit intimately connected to the associated pin. As described a first test word generator 36, Fig. 2, comprises a plurality of EX-OR gates 71, 75, 79, 83, ...., 87 and bi-stables 73, 75, 77, 81, 85, ..., 89, cascade connected as shown. Fig. 2 also shows an example of a circuit under test 50 having both its set of input pins F2, F4, F6, F8, ..., FN and its set of output pins F1, F3, F5, F7, ..., FN, connected to the test word generator via switches controlled by a "mode controller", not described. In operation the test word generator, and the circuit under test, are independently set to datum conditions by signals on lines 52 and 51 respectively. Then the switches coupling the test word generator to the circuit under test are opened by the mode controller and clock pulses, on a line 59, are applied to the test word generator to drive it through a sequence of a fixed number of words. In the particular example shown, which uses a test sequence of 16 words, test word generator is initially set so that its bi-stables all register O's and the states assumed by the bi-stables after each successive clock pulse are shown in Fig. 4. After 16 clock pulses, the test word generator is thus in state "0000", which constitutes the reference word for the particular circuit shown to be under test. If now the circuit under test 50, Fig. 2, which has been assumed -  to be good, is replaced by a similar, but faulty, circuit Fig. 5, the fault being shown as a discontinuity 117, then when the test word generator is driven through 16 words it goes through the sequence shown in Fig. 7, terminating in the word "1100", which differs from the previously derived reference word and indicates that a fault is present somewhere within the circuit under test. Having detected the presence of a fault, the system is then put into its "fault locate" mode. In this mode the output pins of the circuit under test are isolated by the mode controller from the test word generator, and instead a probe 47, Fig.1, is manually positioned on the output pins, one-by-one, and the number of ones appearing at each pin is counted. With the example of a circuit under test shown, output pin F7 will produce a count of 2 in response to a complete cycle of 16 words, if no fault exists in a neighbourhood of pin F7, but a count of zero for the illustrated fault. The Specification also describes two other test word generators Figs. 15 and 20 (not shown) and a second example Fig. 8 (not shown) of a possible circuit that may be tested, but no description of the other blocks shown in Fig. 1 as forming part of the complete system are given.